Diversity transmission method and system

ABSTRACT

The present invention relates to a diversity transmission method and system, wherein a transmission signal is divided into a plurality of subsignals. A first set of the subsignals in transmitted using a first diversity transmission scheme, and a second set of said subsignals is transmitted using a second diversity transmission scheme. Thus, a joint coordination between different types of diversity transmission schemes is proposed so as to achieve a significant capacity increase at a moderate complexity.

FIELD OF THE INVENTION

The present invention relates to a diversity transmission method andsystem for transmitting a transmission signal in a wirelesscommunication system, such as the Universal Mobile TelecommunicationsSystem (UMTS).

BACKGROUND OF THE INVENTION

Wideband Code Division Multiple Access (WCDMA) has been chosen as theradio technology for the paired bands of the UMTS. Consequently, WCDMAis the common radio technology standard for third-generation wide-areamobile communications. WCDMA has been designed for high-data servicesand, more particularly, Internet-based packet-data offering up to 2 Mbpsin indoor environments and over 384 kbps for wide-area applications.

The WCDMA concept is based on a new general structure for all layersbuilt on technologies such as packet-data channels and servicemultiplexing. The new concept also includes pilot symbols and atime-slotted structure which has led to the provision of adaptiveantenna arrays which direct antenna beans at users to provide maximumrange and minimum interference. This is also crucial when implementingwideband technology where limited radio spectrum is available.

The uplink capacity of the proposed WCDMA systems can be enhanced byvarious techniques including multi-antenna reception and multi-userdetection or interference cancellation. Techniques that increase thedownlink capacity have not been developed with the same intensity.However, the capacity demand due to the projected data services (e.g.Internet) burdens more heavily the downlink channel. Hence, it isimportant to find techniques that improve the capacity of the downlinkchannel.

Bearing in mind the strict complexity requirements of terminals, and thecharacteristics of the downlink channel, the provision of multiplereceive antennas is not a desired solution to the downlink capacityproblems Therefore, alternative solutions have been proposed suggestingthat multiple antennas or transmit diversity at the base station willincrease downlink capacity with minor increase of complexity in terminalimplementation.

In third-generation mobile radio systems in general and in particularfor WCDMA systems, the downlink capacity is a bottleneck. This is due tofading of the transmitted signal, wherein the amplitude of the signal issubjected to random fluctuations. To overcome this situation,transmitter antenna diversity has been proposed for the downlinkdirection. Known transmitter diversities schemes can be divided into twocategories, open loop systems and closed loop systems. The differencebetween the open loop and the closed loop systems is that the formersends a feedforward or training information, in order to provide aninformation about the channel at the receiver. On the other hand, thelatter system gets knowledge of the channel at the transmitter side byvirtue of a feedback path from the receiver to the transmitter.Selective Transmit Diversity (STD) is an example of a closed loop systemwhich is easy to implement in digital cellular systems due to thepresence of a permanent feedback connection. Furthermore, systems thatemploy either of the two categories of transmitter diversity are known.

The prior art diversity systems are described e.g. in document U.S. Pat.No. 5,832,044 and in the publications “Fading Resistant Modulation UsingSeveral Transmitter Antennas” by Sousa et al., IEEE Trans. OnCommunications, pp. 1236-1244, October 1997, and “Diversity Transformfor Fading Channels”, by D. Rainish, IEEE Trans. On Communications, pp.1653-1661, December 1996.

In the above prior art systems, all components of a constellation vector(super symbol) are transmitted via either of different antennas,different carrier frequencies, or diferent time slots. However, sincethe optimum decoding complexity grows exponentially with the number ofcomponents of the constellation vector, the transmission capacity islimited. Moreover, a high peak to average ratio results from anincreased constellation size.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a diversitytransmission method and system, by means of which the transmissioncapacity can be increased.

This object is achieved by a diversity transmission method fortransmitting a transmission signal in a wireless communication system,comprising the steps of: dividing the transmission signal into aplurality of subsignals;

-   applying an orthonormal transformation to said plurality of    subsignals;-   transmitting a first set of the subsignals using a first diversity    transmission scheme; and-   transmitting a second set of the subsignals using a second diversity    transmission scheme, the second diversity transmission scheme being    different from the first diversity transmission scheme.

Furthermore, the above object is achieved by a transmitter for adiversity transmission system for transmitting transmission signal in awireless communication system, comprising:

-   dividing means for dividing the transmission signal into a plurality    of subsignals; and-   mtransforming means for applying an orthonormal transformation to    said plurality of subsignals;-   transmitting means for transmitting a first set of the subsignals    using a first diversity transmission scheme, and a second set of the    subsignals using a second diversity transmission scheme different    from the first diversity transmission scheme.

Additionally, the above object is achieved by a receiver for a diversitytransmission system, for receiving a transmission signal in a wirelesscommunication system, comprising:

-   receiving means for receiving a transmission signal comprising a    first set of subsignals transmitted by using a first diversity    transmission scheme, and a second set of subsignals transmitted by    using a second diversity transmission scheme different from the    first diversity transmission scheme; and-   decoding means for decoding the transmission signal by deciding on a    maximum likelihood between the received subsignals and corresponding    estimated subsignals.

Accordingly, a joint coordination between different diversitytransmission types is provided, which results in a significant capacityincrease as compared to previous transmitter diversity schemes based onmultidimensional fading resistant constellations. Thus, an optimumdetection method can be used which makes the optimum decoding complexitygrow linear with the dimension of the constellations.

In a cellular network, a fading resistant transmission scheme can beprovided, where a base station uses M antennas or/and M time slots(regardless of the use a frame orientated power control) or,/and Mcarrier frequencies (for narrow band systems), wherein M denotes thedimension of the signal constellation.

Preferably, the first diversity transmission scheme is a space diversitytransmission scheme, such as a selective transmitter antenna diversity(STD). The second diversity transmission scheme may be a frequency ortime diversity scheme. The original signal constellation may berepresented as a matrix, wherein each row of the matrix corresponds to apoint in a multidimensional constellation. In particular, a complexdiversity transformation may be used, wherein an orthonormaltransformation to a constellation which preserves Euclidean distancesbut improves the resistance to fading may be performed. The orthonormaltransformation may be achieved by a multiplication with a complexmatrix. Preferably, each row of the complex matrix is orthogonal to anyother row, wherein the determinant of the matrix is equal to one. Thecomplex matrix may be obtained based on the upper bound on the symbolerror rate or based on the cut off rate.

Preferably, the diversity transmission method is used in the downlinkdirection of a cellular network.

The transmission signal may be a bit stream and the plurality ofsubsignals may be substreams. In particular, the transmission signal maybe a PSK signal, preferably a QPSK signal which can be represented by avertex in a 2M-dimensional hyper-cube, where M denotes the dimension ofthe signal constellation. In this case, the receiving means may comprisea bank of 2M correlators, wherein M denotes the number of transmissionantennas used in the first diversity transmission scheme.

The wireless communication system may be a WCDMA system, wherein thetransmitter may be arranged in a WCDMA base station and the receiver ina WCDMA mobile station.

Furthermore, the first and second diversity transmission schemes maycomprise an open loop and/or a closed loop system.

Preferably, time slots of frequency carriers used in the seconddiversity transmission scheme are spaced apart to such a degree thatindependent fading is assured. Thereby, the transmissions can becoordinated to mitigate the effects of multi-path Rayleigh fading, andthe receiver can recover the entire M-dimensional transmitted signalconstellation or vector, as long as the signal energy of at least onecoordinate is large enough. In particular, the M-dimensional signalconstellation may be generated by optimizing the bit error rate and thepeak to average ratio for a Rayleigh fading channel. The bit-error-ratemay be further improved by using the STD scheme. This scheme offers asignificant performance improvement over the conventional single antennaN-PSK scheme and other known M-dimensional fading resistingconstellations for a given bit-error-rate. In the downlink direction ofa cellular network, a significant capacity increase is achieved ascompared to uncoded N-PSK and other known M-dimensional fading resistantconstellations.

Preferably, the transmitting means comprises a complex diversitytransformation unit arranged for performing an orthonormaltransformation to a constellation which preserves Euclidean distancesbut improves resistance to fading of an original signal constellationobtained from the dividing means.

Furthermore, the receiver may comprise channel estimation means forperforming a channel estimation used for obtaining the correspondingestimated subsignal.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are intended solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described in greaterdetail on the basis of a preferred embodiment which reference to theaccompanying drawings, in which:

FIG. 1 shows a principle block diagram of a transmission systemaccording to the present invention,

FIG. 2 shows a principle block diagram of a transmitter according to apreferred embodiment of the present invention; and

FIG. 3 shows a principle block diagram of a receiver according to apreferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, the preferred embodiment of the present invention willbe described on the basis of a downlink transmission between a basestation and a mobile station of a cellular network such as the UMTS.

In spectrally efficient transmitter antenna, frequency and timediversity schemes, the information bit stream is divided intosubstreams, wherein each substream is transmitted over a differentantenna, a different frequency, or a different time slot. Takingjointly, the transmission of a set of symbols can be viewed as thetransmission of a super symbol. In the case of a QPSK transmission, thesuper symbol can be represented by a vertex in a 2M-dimensionalhyper-cube, where M denotes the number of antennas, frequencies, or timeslots.

According to the preferred embodiment of the present invention, awideband system is considered such that the use of multiple carriers isnot appropriate and will not be described in detail. However, thepresent invention is not restricted to wideband systems.

The WCDMA system operates at a low signal to noise ratio. Therefore,optimal signaling constellations for N-PSK modulations which are fadingresistant at low signal to noise ratios are required.

The transmitter transmits a sequence of symbols from a fixed symbolalphabet. As already mentioned, each symbol may be represented as avector in an M-dimensional vector space. Thus, each vector has Mcomponents. According to the present invention, the transmission of theM components is combined in different antennas and different time slots.Furthermore, in case of narrowband systems, a combination with differentfrequency carriers can be used for transmitting the M components. Inparticular, the constellation is obtained from an M-dimensionalhyper-cube based on an orthogonal transformation. However, theseparation between constellation points may be further maximized andnone of the points are superimposed. The result is a much betterperformance over fading channels.

FIG. 1 shows a principle block diagram of the diversity transmissionsystem according to a present invention. The system comprises Mtransmitter antennas (not shown) for transmitting a transmission signalfrom a base station to a mobile terminal, and a single receiver antenna(not shown). Thus, the received base band signal is obtained by thefollowing equation: $\begin{matrix}{{r(t)} = {\sum\limits_{i = 1}^{M}{x_{i}{s_{i}(t)}{\sum\limits_{j = 1}^{L}\left( {\alpha_{i}^{j} + {n_{i}^{j}(t)}} \right)}}}} & (1)\end{matrix}$Between the receiver antenna and each transmitter antennas there are Lmulti-paths, wherein the symbol α_(l) ^(j) denotes the Rayleigh fadingof the j-th multi-path of the i-th transmit antenna at the receiver,x_(i) represents the N-PSK transformed signal on the i-th antenna,s_(i)(t) denotes a bandlimited pulse, where s_(i)(t), s_(k)(t) areassumed to be orthogonal for i≈k. similarly, n_(i) ^(j) (t) denotes theadded AWGN (Additive Wide Gaussian Noise) with power spectral densityNo/2.

An independent fading can be assumed if the transmitter antennas or timeslots of frequency carriers are sufficiently spaced apart.

According to the present invention, the receiver 4 is capable ofestimating the fading amplitude of each link. This is possible, if thefading amplitudes vary slowly over time. If the fading amplitudes varyquickly over time, it is expected that the receiver performance degradesdue to estimation errors.

In the transmitter, a complex diversity transformation unit 1 isprovided for performing a diversity transformation of an input signalconstellation set which can be represented as a matrix. The interleaver2 and deinterleaver 3 shown in FIG. 1 are not specific to the diversitytransformation. They relate to the usual interleaving required forsystems with forward error correction capabilities. In such systems, itis necessary to assure that fading amplitudes are uncorrelated. Thedelay introduced by the interleaver depends on the giving service andthe general fading characteristics.

The original signal constellation is represented as a matrix Z, whereeach row corresponds to a point in the M-dimensional constellationcorresponding to M input (encoded) symbols. Given the M-dimensionalconstellation of Q=N^(M) points, a transformation is applied by thecomplex diversity transformation unit 1, such that the Euclideandistances between the constellation points are preserved, but theconstellations resistance to fading is improved. The requirement thatthe transformation preserves the Euclidean distances between points andnorms is imposed to thereby assure that the performance of the newconstellation in the AWGN channel is not degraded. Such orthonormaltransformations are called isometries.

For example, in case of a BPSK or a QPSK system, the originalconstellations for M=2 are given by ${Z_{BPSK} = \begin{bmatrix}1 & 1 \\1 & {- 1} \\{- 1} & {- 1} \\{- 1} & 1\end{bmatrix}},{Z_{QPSK} = \begin{bmatrix}{1 + j} & {1 + j} \\{1 + j} & {1 - j} \\{1 + j} & {{- 1} - j} \\{1 + j} & {{- 1} + j} \\{1 - j} & {1 + j} \\{1 - j} & {1 - j} \\{1 - j} & {{- 1} - j} \\{1 - j} & {{- 1} + j} \\{{- 1} - j} & {1 + j} \\{{- 1} - j} & {1 - j} \\{{- 1} - j} & {{- 1} - j} \\{{- 1} - j} & {{- 1} + j} \\{{- 1} + j} & {1 + j} \\{{- 1} + j} & {1 - j} \\{{- 1} + j} & {{- 1} - j} \\{{- 1} + j} & {{- 1} + j}\end{bmatrix}}$These constellation matrixes are multiplied in the complex diversitytransformation unit 1 by an orthonormal M×M matrix A_(M) to therebypreserve the distance between vectors, and the energy. The transformedconstellation x is given byX=ZA_(M)  (2)To be orthonormal, the complex matrix AM must fulfill the followingconditions:

-   (1) each row is orthogonal to any other row;-   (2) the determinant of the matrix is equal to one.    According to the invention, the orthonormal complex matrixes for M=2    and M=2^(n) are generally given by $\begin{matrix}    {{\overset{\_}{A_{2}} = \frac{A_{2}}{{A_{2}}^{1/M}}},{A_{2} = \begin{bmatrix}    {\mathbb{e}}^{j\phi} & {\mathbb{e}}^{- {j\phi}} \\    {- {\mathbb{e}}^{- {j\phi}}} & {\mathbb{e}}^{j\phi}    \end{bmatrix}},{{A_{2}} = {{\det\quad\left( A_{2} \right)} = {{2\quad{\cos\left( {2\phi} \right)}\overset{\_}{A_{2n}}} = \frac{A_{2n}}{{A_{2n}}^{1/M}}}}},{A_{2n} = \begin{bmatrix}    A_{n} & A_{n} \\    A_{n} & {- A_{n}}    \end{bmatrix}},{{A_{2n}} = {{\det\left( A_{2n} \right)} = {f(\phi)}}}} & (3)    \end{matrix}$    wherein φ denotes the angle that must be chosen in order to minimize    the error probability in fading channels. This, however, constitutes    an untractable problem in mathematics. Therefore, two other    suboptimal approaches can be used, i.e. the upperbound on the symbol    error rate and the cutoff rate, wherein the vector is assumed to be    part of a random code with infinite length in which all vectors are    independent. The upperbound on the symbol error rate is described    e.g. in “Introduction to Trellis-Coded Modulation with Applications”    by E. Biglieri et al., Macmillan Pub., 1993, chapter 9, and is given    by $\begin{matrix}    {{P\left( {x->\underset{\_}{x}} \right)} = {{Min}\quad{\forall{\underset{x \neq \underset{\_}{x}}{\left( {x,\underset{\_}{x}} \right)}{\underset{i = 1}{\overset{M}{\quad\prod}}\quad\frac{1}{1 + {\frac{E_{s}}{4N_{0}}{{x_{i} - \underset{\_}{x_{i}}}}^{2}}}}}}}} & (4)    \end{matrix}$    and the cutoff rate is described e.g. in “Diversity Transform for    Fading Channels” by D. Rainish, IEEE Transaction On Communications,    pp. 1653-1661, December 1996. and is given by $\begin{matrix}    {R_{0} = {{\log_{2}N} - {\frac{1}{M}{\log_{2}\left\lbrack {\forall{\underset{x \neq \underset{\_}{x}}{\left( {x,\underset{\_}{x}} \right)}{\prod\limits_{i = 1}^{M}\quad\frac{1}{1 + {\frac{E_{s}}{4N_{0}}{{x_{i} - \underset{\_}{x_{i}}}}^{2}}}}}} \right\rbrack}}}} & (5)    \end{matrix}$    wherein N denotes the dimension of the modulation, e.g. N=4 for QPSK    modulation.

Comparing the two above equations (4) and (5) it can be seen that theyare somehow related. The main difference lies in the minimum operatoronly used in the bit error rate upperbound. This criteria is optimum fora use of the scheme in a high signal to noise ratio environment. In thisinvention, however, the interesting signal to noise ratio (SNR)comprises small values of E_(s)/N_(O). Thus, the cutoff rate ispreferably chosen, because it considers all pairs of (x,x), wherein xindicates the transmitted vector and x denotes the super symbol pickedup in the receiver 4. There are certain values of φ which cannot beused. Those values must be avoided. For N-PSK modulations, theacceptable angles are defined by $\begin{matrix}{{\phi \neq {\pi/N}},{\phi < {\pi/N}}} & (6)\end{matrix}$To be fading resistant, any two points of the signal constellationshould have a large number of components which differ significantly. Forevery M and in particular for M=2, it is important that the determinantof the complex matrix A_(M) is minimized, such chat a large number ofcomponents differ significantly and a better performance of the schemeis achieved, However, to obtain low determinant values, the limit givenby equation (6) must be approached, which originates signalingconstellation points close to zero and high peak to average amplituderatios. The search for an optimal angle φ can be made exhaustively forsmall discredisation intervals, e.g. 1°, because only one angle has tobe optimized. The optimal interval for the angle φ is [π/8, π/6]. In thepreferred embodiment, an angle φ=π/6 has been chosen. For this angle,the determinant of the complex matrix A₂ is equal to one.

In general, the performance results obtained by a complex orthogonalmatrix are better than those obtained by a real orthonormal matrix forM=2 and M=4.

According to the preferred embodiment of the present invention, aselective transmitter antenna diversity (STD) is combined with thecomplex diversity transformation (CDT). Thereby, a diversity of anyorder can be obtained. For instance, a diversity of order 8 can beobtained e.g. by using a complex diversity transformation of order 4(time diversity) and an STD with 2 antennas, or by using a complexdiversity transformation of order 2 and a STD diversity with 4 antennas.

FIG. 2 shows a principal block diagram of a transmitter which may beused in a base station and in which a combined CDT and STD areperformed. According to FIG. 2 the transmitter comprises a coding unit10 arranged for generating the signal constellation matrix Z based onreceived input symbols to be transmitted to a mobile station. Thegenerated constellation matrix Z is supplied to a complex diversitytransformation unit 11 which performs a multiplication of theconstellation matrix Z with the orthonormal matrix A_(M), as defined inthe equation (2). In particular, the coding unit 10 and the complexdiversity transformation unit 11 may be realize, by correspondingdigital processing circuits or by a central processing unit controlledon the basis of a corresponding control program. The obtainedtransformed signal constellation matrix X is supplied to a transmittingunit Tx 12, wherein each column of the transformed constellation matrixX corresponds to a respective one of a plurality of transmissionantennas A1, A2, . . . AM, such that a first set of subsignals orsubsymbols (correspoonding to the matrix columns) are transmitted viarespective different ones of the transmission antennas A1 to AM, and asecond set of subsignals or subsymbols (corresponding to matrix rows)are transmitted in respective different time slots.

FIG. 3 shows a corresponding receiver of the transmission system, whichmay be provided in a mobile station of a cellular network. In thepresent case, a QPSK modulation is used for the transmission, whereinthe receiver is a QPSK optimum receiver consisting of a bank of 2Mintegrators (or correlators) 4110, 4111, 4120, 4121, . . . 41M0, 41M1.

The radio signals transmitted from the transmission antennas A1 to AMare received via a single receiving antenna by a receiving unit Rx 40 ofthe receiver, and an in-phase component and a quadrature component areobtained by multiplying the received signal with a sine signal and acosine signal, respectively, of the carrier frequency. The in-phase andquadrature components are each supplied to M processing channels, wherea detection is performed based on a multiplication with respectivebandlimited pulse signals s₁(t), s₂(t), . . . s_(M)(t). The detectedreceived signals are supplied to respective ones of the integrators 4110to 41M1. In the present preferred embodiment, only coherent demodulatorsare considered. For a single path Rayleigh fading channel (L=1,subscript j dropped), the output of the i-th integrator (correlator) isgiven by $\begin{matrix}{{y_{i} = {{\overset{T}{\int\limits_{0}}{{r(t)}{s_{i}(t)}{\mathbb{d}t}}} = {{\alpha_{i}x_{i}E_{s}} + \eta_{i}}}},{E_{s} = {\int_{0}^{T}\quad{{s_{i}^{2}(t)}{\mathbb{d}t}}}}} & (7)\end{matrix}$wherein η_(i) (l≦i≦M) denotes an uncorrelated zero-mean Gaussian randomvariable with variance N₀E_(s), and wherein T denotes the time period ofa received symbol. Thus, most of the energy of the signal s_(i)(t) iscontained in the interval [0, T].

The outputs of the integrators 411C, 4120, . . . , 41M0 of the in-phasecomponent and the integrators 4111, 4121, . . . , 41M1 of the quadraturecomponent are combined by respective combining circuits 421 to 42M whichoutput the components y₁ to y_(M) of the received vector y. The receivedvector y=(y₁, . . . y_(M)) is supplied to a decision device such as aminimum distance decoder 43 which estimates the transmitted vectorx=(x₁, . . . x_(M)). Furthermore, a channel estimator 44 is provided forestimating fading amplitudes α_(i) and for supplying the estimatedfading amplitudes α_(i) to the minimum distance decoder 43. The minimumdistance decoder 43 selects a super symbol x=(x ₁, . . . x_(M)) which isan element of an M-dimensional constellation. The selection is performedin such a manner that the following equation is satisfied$\begin{matrix}{{{Min}{\sum\limits_{i = 1}^{M}\quad{{{y_{i}/E_{s}} - {\alpha_{i}\underset{\_}{x}}}}^{2}}},{\forall\left( {x,\underset{\_}{x}} \right)}} & (8)\end{matrix}$A symbol detecting error occurs, when x≠x. Thus, the receiver is amaximum likelihood receiver arranged to choose between N^(M) (N is thesize of the alphabet) possible different combinations of (x,x).

The communication links between the transmitting antennas A1 to AM, andthe receiving antenna are not generally line-of-sight links. In general,a multi-path Rayleigh fading model is assumed. The fading amplitudesα_(i) ^(j) are modelled as independent and identically distributedRayleigh random variables, wherein the probability density function isgiven byƒ(α)=2α exp(−α²), α≧0  (9)

It is to be noted that the interleave 2 and the deinterleaver 3 shown inFIG. 1 and required for forward error correction capabilities are notshown in the transmitter and the receiver according to FIGS. 2 and 3,respectively.

The present invention is not restricted to a combination of CDT withSTD. Any combination of different diversity schemes can be used, whereina combination of a space diversity scheme such as STD with timediversity schemes such as CDT, RDT (Real Diversity Transformation), maybe applied.

The reference probability of the bit error rate for uncoded widebandsystems is P_(b)=4×10⁻². For this reference P_(b), a gain of 2 dB can beachieved between CDT and RDT without STD. If STD is combined with otherdiversity transformations, a gain of 1.5 dB is achieved between CDT andRDT. Compared to a single STD, the combination of CDT with STD providesan additional gain of 2.1 dB. For all diversity transformations, anoptimal angle φ=π/6 has been obtained.

For higher diversity orders, such as M=4, CDT continues to provideadditional gain over RDT with and without STD, however, now these gainsare not so significant. When CDT (with diversity order 2) and STD (withdiversity order 2) are combined, the equivalent diversity order is 4.Another way to achieve this diversity order is CDT with diversity order4. The comparison between these two cases indicates that CDT+STD leadsto a better performance and should therefore be chosen.

The minimum distance decoder 43 shown in FIG. 3 is able to avoid theexponential growth of the decoding complexity, when the minimum distanceis chosen between (x,x), as given by equation (8). This can be gatheredfrom the following equation|y₁/E_(s)−α₁ x ₁|²,∀(x₁,x ₁)∴|y₂/E_(s)−α₂ x ₂|²,∀(x₂ x ₂)∴. . .|y_(M)/E_(s)−α_(M) x _(M)|²,∀(x_(M), x _(M))  (10)Since the metrics is positive and additive, it is better to compute theminimum distance individually for each link i and decide individually onthe transmitted symbols.

As an example, a 4-PSK signal with diversity order M=4 is considered.Based on the equation (10) the following result is obtained.d ₁ =|y ₁ /E _(s)−α₁(1+j)|² , d ₂ =|y ₁ /E _(s)−α₁(1−j)|² ,d ₃ =|y ₁ /E_(s)−α₁(−1−j)|² ,d ₄ =|y ₁ /E _(s)−α₁(−1+j)|²  (11)

Accordingly, the minimum is chosen for all d_(n)(l≦n≦N), which leads tothe decision on x₁. Next, the decision is made as to x₂, based on theminimum of N metrics, and so on, until a decision is made on x_(M), alsobased on N metrics, wherein N=4. Thus, M decisions are performed basedon N metrics. Thus, N×M metrics have to be computed, instead of N^(M) asin the known solutions.

The present inventions can be implemented in a variety of ways. Acombination of spectrally efficient transmitter tire diversity of orderM₁ with a selective transmitter antenna diversity (STD) of order M₂ ispreferred to achieve a total diversity order of M=M₁×M₂. For narrowbandsystems, the present invention can be implemented as a spectrallyefficient transmitter frequency diversity scheme in combination withSTD, so as to increase the order of the diversity.

The present invention can be applied to improve the performance of thephysical layer of the UMTS UTRA/FDD (UMTS Radio Access/FrequencyDivision Duplex) components. Alternatively, it may be implemented in thephysical layer of UMTS UTRA/TDD (Time Division Duplex) components. Ingeneral, the present invention can be implemented in any transmissionlink of any digital cellular network to thereby increase the capacity ofthat link. Therefore, the above description of the preferred embodimentand the accompanying drawings are only intended to illustrate thepresent invention. The preferred embodiment of the invention may varywithin the scope of the attached claims.

In summary, the present invention relates to a diversity transmissionmethod and system, wherein a transmission signal is divided into aplurality of subsignals. A first set of the subsignals is transmittedusing a first diversity transmission scheme, and a second set of thesubsignals is transmitted using a second diversity transmission scheme,Thus, a joint coordination between different types of diversitytransmission schemes is proposed so as to achieve a significant capacityincrease at a moderate complexity.

Thus, while there have been shown and described and pointed outfundamental novel features of the present invention as applied to apreferred embodiment thereof, it will be understood that variousomissions and substitutions and changes in the form and details of thedevices described and illustrated, and in their operation, and of themethods described may be made by those skilled in the art withoutdeparting from the spirit of the present invention. For example, it isexpressly intended that all combinations of those elements and/or methodsteps which perform substantially the same function in substantially thesame way to achieve the same results are within the scope of theinvention. Substitutions of elements from one described embodiment toanother are also fully intended and contemplated. It is the intention,therefore, to be limited only as indicated by the scope of the claimsappended hereto.

1. A diversity transmission method for transmitting a transmissionsignal in a wireless communication system, comprising the steps of: a)dividing said transmission signal into a plurality of subsignals; b)applying an orthonormal transformation to said plurality of subsignals;c) transmitting a first set of subsignals using a first diversitytransmission scheme; and d) transmitting a second set of said subsignalsusing a second diversity transmission scheme, said second diversitytransmission scheme being different from said first diversitytransmission scheme.
 2. A method according to claim 1, wherein saidfirst diversity transmission scheme is a space diversity transmissionscheme.
 3. A method according to claim 2, wherein said second diversitytransmission scheme is a frequency or time diversity scheme.
 4. A methodaccording to claim 2, wherein said diversity transmission method is usedin a downlink transmission of a cellular network.
 5. A method accordingto claim 2, wherein said transmission signal is a bit stream and saidplurality of subsignals are substreams.
 6. A method according to claim2, wherein said wireless communication system is a WCDMA system.
 7. Amethod according to claim 2, wherein said first and second diversitytransmission schemes comprise at least one of an open loop and a closedloop system.
 8. A method according to claim 2, wherein time slots offrequency carriers used in said second diversity transmission scheme arespaced apart to such a degree that independent fading is assured.
 9. Amethod according to claim 2, wherein said transmission signal comprisesa signal constellation generated by optimizing the bit error rate andthe peak to average ratio for a Rayleigh fading channel.
 10. A methodaccording to claim 2, wherein said space diversity transmission schemeis a selective transmitter antenna diversity scheme.
 11. A methodaccording to claim 10, wherein said second diversity transmission schemeis a frequency or time diversity scheme.
 12. A method according to claim10, wherein said diversity transmission method is used in a downlinktransmission of a cellular network.
 13. A method according to claim 10,wherein said transmission signal is a bit stream and said plurality ofsubsignals are substreams.
 14. A method according to claim 10, whereinsaid wireless communication system is a WCDMA system.
 15. A methodaccording to claim 10, wherein said first and second diversitytransmission schemes comprise at least one of an open loop and a closedloop system.
 16. A method according to claim 10, wherein time slots offrequency carriers used in said second diversity transmission scheme arespaced apart to such a degree that independent fading is assured.
 17. Amethod according to claim 10, wherein said transmission signal comprisesa signal constellation generated by optimizing the bit error rate andthe peak to average ratio for a Rayleigh fading channel.
 18. A methodaccording to claim 1, wherein said second diversity transmission schemeis a frequency or time diversity scheme.
 19. A method according to claim18, wherein said diversity transmission method is used in a downlinktransmission of a cellular network.
 20. A method according to claim 18,wherein said transmission signal is a bit stream and said plurality ofsubsignals are substreams.
 21. A method according to claim 18, whereinsaid wireless communication system is a WCDMA system.
 22. A methodaccording to claim 18, wherein said first and second diversitytransmission schemes comprise at least one of an open loop and a closedloop system.
 23. A method according to claim 18, wherein time slots offrequency carriers used in said second diversity transmission scheme arespaced apart to such a degree that independent fading is assured.
 24. Amethod according to claim 18, wherein said transmission signal comprisesa signal constellation generated by optimizing the bit error rate andthe peak to average ratio for a Rayleigh fading channel.
 25. A methodaccording to claim 18, wherein said second diversity transmission schemeis a complex diversity transform scheme.
 26. A method according to claim25, wherein said diversity transmission method is used in a downlinktransmission of a cellular network.
 27. A method according to claim 25,wherein said transmission signal is a bit stream and said plurality ofsubsignals are substreams.
 28. A method according to claim 25, whereinsaid wireless communication system is a WCDMA system.
 29. A methodaccording to claim 25, wherein said first and second diversitytransmission schemes comprise at least one of an open loop and a closedloop system.
 30. A method according to claim 25, wherein time slots offrequency carriers used in said second diversity transmission scheme arespaced apart to such a degree that independent fading is assured.
 31. Amethod according to claim 25, wherein said transmission signal comprisesa signal constellation generated by optimizing the bit error rate andthe peak to average ratio for a Rayleigh fading channel.
 32. A methodaccording to claim 25, wherein said complex diversity transform schemecomprises an orthonormal transformation to a constellation whichpreserves Euclidean distances.
 33. A method according to claim 32,wherein said diversity transmission method is used in a downlinktransmission of a cellular network.
 34. A method according to claim 32,wherein said transmission signal is a bit stream and said plurality ofsubsignals are substreams.
 35. A method according to claim 32, whereinsaid wireless communication system is a WCDMA system.
 36. A methodaccording to claim 32, wherein said first and second diversitytransmission schemes comprise at least one of an open loop and a closedloop system.
 37. A method according to claim 32, wherein time slots offrequency carriers used in said second diversity transmission scheme arespaced apart to such a degree that independent fading is assured.
 38. Amethod according to claim 32, wherein said transmission signal comprisesa signal constellation generated by optimizing the bit error rate andthe peak to average ratio for a Rayleigh fading channel.
 39. A methodaccording to claim 32, wherein an original signal constellationrepresented as a matrix is used, and wherein each row of said matrixcorresponds to a point in a multidimensional constellation.
 40. A methodaccording to claim 32, wherein said orthonormal transformation isachieved by a multiplication with a complex matrix.
 41. A methodaccording to claim 40, wherein said diversity transmission method isused in a downlink transmission of a cellular network.
 42. A methodaccording to claim 40 wherein said transmission signal is a bit streamand said plurality of subsignals are substreams.
 43. A method accordingto claim 40, wherein said wireless communication system is a WCDMAsystem.
 44. A method according to claim 40, wherein said first andsecond diversity transmission schemes comprise at least one of an openloop and a closed loop system.
 45. A method according to claim 40,wherein time slots of frequency carriers used in said second diversitytransmission scheme are spaced apart to such a degree that independentfading is assured.
 46. A method according to claim 40, wherein saidtransmission signal comprises a signal constellation generated byoptimizing the bit error rate and the peak to average ratio for aRayleigh fading channel.
 47. A method according to claim 40, whereineach row of said complex matrix is orthogonal to any other row, andwherein the determinant of said matrix is equal to one.
 48. A methodaccording to claim 47, wherein said complex matrix is obtained based onan upperbound on the symbol error rate or based on a cutoff rate.
 49. Amethod according to claim 47, wherein said diversity transmission methodis used in a downlink transmission of a cellular network.
 50. A methodaccording to claim 47, wherein said transmission signal is a bit streamand said plurality of subsignals are substreams.
 51. A method accordingto claim 47, wherein said wireless communication system is a WCDMAsystem.
 52. A method according to claim 47, wherein said first andsecond diversity transmission schemes comprise at least one of an openloop and a closed loop system.
 53. A method according to claim 47,wherein time slots of frequency carriers used in said second diversitytransmission scheme are spaced apart to such a degree that independentfading is assured.
 54. A transmitter according to claim 47, wherein saidsecond diversity transmission scheme is a time or frequency diversitytransmission scheme using a plurality of time slots or carrierfrequencies.
 55. A method according to claim 40, wherein said complexmatrix is obtained based on an upperbound on the symbol error rate orbased on a cutoff rate.
 56. A method according to claim 53, wherein saidorthonormal transformation is achieved by a multiplication with acomplex matrix.
 57. A method according to claim 53, wherein saiddiversity transmission method is used in a downlink transmission of acellular network.
 58. A method according to claim 53, wherein saidtransmission signal is a bit stream and said plurality of subsignals aresubstreams.
 59. A method according to claim 55, wherein said first andsecond diversity transmission schemes comprise at least one of an openloop and a closed loop system.
 60. A method according to claim 53,wherein said first and second diversity transmission schemes comprise atleast one of an open loop and a closed loop system.
 61. A methodaccording to claim 55, wherein said transmission signal comprises asignal constellation generated by optimizing the bit error rate and thepeak to average ratio for a Rayleigh fading channel.
 62. A methodaccording to claim 53, wherein said transmission signal comprises asignal constellation generated by optimizing the bit error rate and thepeak to average ratio for a Rayleigh fading channel.
 63. A methodaccording to claim 40, wherein said complex matrix is obtained based onan upperbound on the symbol error rate or based on a cutoff rate.
 64. Amethod according to claim 63, wherein said diversity transmission methodis used in a downlink transmission of a cellular network.
 65. A methodaccording to claim 63, wherein said transmission signal is a bit streamand said plurality of subsignals are substreams.
 66. A method accordingto claim 63, wherein said wireless communication system is a WCDMAsystem.
 67. A method according to claim 63, wherein said first andsecond diversity transmission schemes comprise at least one of an openloop and a closed loop system.
 68. A method according to claim 63,wherein time slots of frequency carriers used in said second diversitytransmission scheme are spaced apart to such a degree that independentfading is assured.
 69. A method according to claim 63, wherein saidtransmission signal comprises a signal constellation generated byoptimizing the bit error rate and the peak to average ratio for aRayleigh fading channel.
 70. A method according to claim 1, wherein saiddiversity transmission method is used in a downlink transmission of acellular network.
 71. A method according to claim 70, wherein saidtransmission signal is a bit stream and said plurality of subsignals aresubstreams.
 72. A method according to claim 70, wherein said wirelesscommunication system is a WCDMA system.
 73. A method according to claim70, wherein said first and second diversity transmission schemescomprise at least one of an open loop and a closed loop system.
 74. Amethod according to claim 70, wherein time slots of frequency carriersused in said second diversity transmission scheme are spaced apart tosuch a degree that independent fading is assured.
 75. A method accordingto claim 70, wherein said transmission signal comprises a signalconstellation generated by optimizing the bit error rate and the peak toaverage ratio for a Rayleigh fading channel.
 76. A method according toclaim 1, wherein said transmission signal is a bit stream and saidplurality of subsignals are substreams.
 77. A method according to claim76, wherein said wireless communication system is a WCDMA system.
 78. Amethod according to claim 76, wherein said first and second diversitytransmission schemes comprise at least one of an open loop and a closedloop system.
 79. A method according to claim 76, wherein time slots offrequency carriers used in said second diversity transmission scheme arespaced apart to such a degree that independent fading is assured.
 80. Amethod according to claim 76, wherein said transmission signal comprisesa signal constellation generated by optimizing the bit error rate andthe peak to average ratio for a Rayleigh fading channel.
 81. A methodaccording to claim 76, wherein said transmission signal is a QPSK signalwhich can be represented by a vertex in a 2M-dimensional hyper-cube,where M denotes the dimension of a signal constellation.
 82. A methodaccording to claim 81, wherein said wireless communication system is aWCDMA system.
 83. A method according to claim 81, wherein said first andsecond diversity transmission schemes comprise at least one of an openloop and a closed loop system.
 84. A method according to claim 81,wherein time slots of fequency carriers used in said second diversitytransmission scheme are spaced apart to such a degree that independentfading is assured.
 85. A method according to claim 81, wherein saidtransmission signal comprises a signal constellation generated byoptimizing the bit error rate and the peak to average ratio for aRayleigh fading channel.
 86. A method according to claim 1, wherein saidwireless communication system is a WCDMA system.
 87. A method accordingto claim 86, wherein said first and second diversity transmissionschemes comprise at least one of an open loop and a closed loop sysem.88. A method according to claim 86, wherein said first and seconddiversity transmission schemes comprise at least one of an open loop anda closed loop system.
 89. A method according to claim 86, wherein saidtransmission signal comprises a signal constellation generated byoptimizing the bit error rate and the peak to average ratio for aRayleigh fading channel.
 90. A method according to claim 1, wherein saidfirst and second diversity transmission schemes comprise at least one ofan open loop and a closed loop system.
 91. A method according to claim90, wherein time slots of frequency carriers used in said seconddiversity transmission scheme are spaced apart to such a degree thatindependent fading is assured.
 92. A method according to claim 90,wherein said transmission signal comprises a signal constellationgenerated by optimizing the bit error rate and the peak to average ratiofor a Rayleigh fading channel.
 93. A method according to claim 1,wherein time slots of frequency carriers used in said second diversitytransmission scheme are spaced apart to such a degree that independentfading is assured.
 94. A method according to claim 93, wherein saidtransmission signal comprises a signal constellation generated byoptimizing the bit error rate and the peak to average ratio for aRayleigh fading channel.
 95. A method according to claim 1, wherein saidtransmission signal comprises a signal constellation generated byoptimizing the bit error rate and the peak to average ratio for aRayleigh fading channel.
 96. A transmitter for a diversity transmissionsystem for transmitting a transmission signal in a wirelesscommunication system, comprising: a)dividing means (10) adapted todivide said transmission signal into a plurality of subsignals;b)transforming means (11) adapted to apply an orthonormal transformationto said plurality of subsignals; and c)transmitting means (12) adaptedto transmit a first set of said subsignals using a first diversitytransmission scheme, and a second set of said subsignals using a seconddiversity transmission scheme different from said first diversitytransmission scheme.
 97. A transmitter according to claim 96, whereinsaid first diversity transmission scheme is a space diversitytransmission scheme using a plurality of transmission antennas (A1-AM).98. A transmitter according to claim 97, wherein said second diversitytransmission scheme is a time or frequency diversity transmission schemeusing a plurality of time slots or carrier frequencies.
 99. Atransmitter according to claim 97, wherein said transforming meanscomprises a complex diversity transformation unit (11) arranged forperforming an orthonormal transformation to constellation whichpreserves Euclidean distances.
 100. A transmitter according to claim 97,wherein said transmitter is arranged in a WCDMA base station.
 101. Atransmitter according to claim 96, wherein said second diversitytransmission scheme is a time or frequency diversity transmission schemeusing a plurality of time slots or carrier frequencies.
 102. Atransmitter according to claim 101, wherein said transforming meanscomprises a complex diversity transformation unit (11) arranged forperforming an orthonormal transformation to constellation whichpreserves Euclidean distances.
 103. A transmitter according to claim101, wherein said transmitter is arranged in a WCDMA base station. 104.A transmitter according to claim 96, wherein said transforming meanscomprises a complex diversity transformation unit (11) arranged forperforming an orthonormal transformation to constellation whichpreserves Euclidean distances.
 105. A transmitter according to claim104, wherein said transmitter is arranged in a WCDMA base station. 106.A transmitter according to claim 96, wherein said transmitter isarranged in a WCDMA base station.
 107. A receiver for a diversitytransmission system, for receiving a transmission signal in a wirelesscommunication system, comprising: a) receiving means (40, 4110, 4111,4120, 4l21, 4lM0, 4lM1, 421, 422, . . . 42M) adapted to receive atransmission signal comprising a first set of subsignals transmitted byusing a first diversity transmission scheme, and a second set ofsubsignals transmitted by using a second diversity transmission schemedifferent from said first diversity transmission scheme; and b) decodingmeans (43) adapted to decode said transmission signal by deciding on amaximum likelihood between said received subsignals and correspondingestimated subsignals.
 108. A receiver according to claim 107, furthercomprising channel estimation means (44) adapted to perform a channelestimation used for obtaining said corresponding estimated subsignals.109. A receiver according to claim 108, wherein said first diversitytransmission scheme is a space diversity transmission scheme.
 110. Areceiver according to claim 108, wherein said second diversity scheme isa time or frequency diversity scheme.
 111. A receiver according to claim108, wherein said transmission signal is a QPSK signal and saidreceiving means comprises a bank of 2M correlators, wherein M denotesthe number of transmission antennas used in said first diversitytransmission scheme.
 112. A receiver according to claim 108, whereinsaid receiver is arranged in a mobile WCDMA terminal of cellularnetwork.
 113. A receiver according to claim 107, wherein said firstdiversity transmission scheme is a space diversity transmission scheme.114. A receiver according to claim 113, wherein said second diversityscheme is a time or frequency diversity scheme.
 115. A receiveraccording to claim 113, wherein said transmission signal is a QPSKsignal and said receiving means comprises a bank of 2M correlators,wherein M denotes the number of transmission antennas used in said firstdiversity transmission scheme.
 116. A receiver according to claim 113,wherein said receiver is arranged in a mobile WCDMA terminal of cellularnetwork.
 117. A receiver according to claim 113, wherein said spacediversity transmission scheme is a selective transmitter antennadiversity scheme.
 118. A receiver according to claim 117, wherein saidsecond diversity scheme is a time or frequency diversity scheme.
 119. Areceiver according to claim 117, wherein said transmission signal is aQPSK signal and said receiving means comprises a bank of 2M correlators,wherein M denotes the number of transmission antennas used in said firstdiversity transmission scheme.
 120. A receiver according to claim 117,wherein said receiver is arranged in a mobile WCDMA terminal of cellularnetwork.
 121. A receiver according to claim 107, wherein said seconddiversity scheme is a time or frequency diversity scheme.
 122. Areceiver according to claim 121, wherein said transmission signal is aQPSK signal and said receiving means comprises a bank of 2M correlators,wherein M denotes the number of transmission antennas used in said firstdiversity transmission scheme.
 123. A receiver according to claim 121,wherein said receiver is arranged in a mobile WCDMA terminal of cellularnetwork.
 124. A receiver according to claim 121, wherein said time orfrequency diversity scheme is a complex diversity transformation scheme.125. A receiver according to claim 124, wherein said transmission signalis a QPSK signal and said receiving means comprises a bank of 2Mcorrelators, wherein M denotes the number of transmission antennas usedin said first diversity transmission scheme.
 126. A receiver accordingto claim 124, wherein said receiver is arranged in a mobile WCDMAterminal of cellular network.
 127. A receiver according to claim 107,wherein said transmission signal is a QPSK signal and said receivingmeans comprises a bank of 2M correlators, wherein M denotes the numberof transmission antennas used in said first diversity transmissionscheme.
 128. A receiver according to claim 127, wherein said receiver isarranged in a mobile WCDMA terminal of cellular network.
 129. A receiveraccording to claim 107, wherein said receiver is arranged in a mobileWCDMA terminal of cellular network.